The invention relates to scanning of laser beams onto work surfaces. More particularly, it relates to improved scanning of a reshaped, concentrated laser beam onto a workpiece utilizing rotating spherical mirrors to achieve surface heat treating. The invention encompasses both a method and an apparatus for such scanning.
Laser heat treating of metals has become an accepted and proven industrial process and has expanded into many industries. The use of lasers for heat treating has provided unique solutions to many difficult problems and has replaced such hardening methods as chrome plating in many instances. Its main utility is the ability selectively to harden specific areas of a part without affecting the surrounding area, which often is not desired to be hardened. This can be accomplished because of the unique capability of a laser to concentrate an extremely high energy in a well-defined area. The beam striking the surface to be heat treated can be shaped to give the proper heat treated profile and also to maximize coverage rates.
In laser heat treating, the ability to change the size and shape of the beam is very important. There have been basically four separate techniques for shaping the beam:
1. Reimaging the beam using spherical optics enables changing the absolute size of the beam but not its general shape or power distribution. PA1 2. Reimaging the beam using cylindrical optics enables changing absolute size and shape of the beam but not its general power distribution. PA1 3. Integration of the beam using a segmented aperture mirror and spherical optics results in a change of the absolute size and shape of the beam and an averaging of spatial variations in power density. PA1 4. Scanning the beam with a single vibrating concave mirror enables changing the shape and the absolute size of the beam and also smoothes spatial variations in power density.
Techniques 1 and 2 are limited to specific applications where the resultant uneven power distribution is acceptable. Technique 1 is by far the simplest and least expensive way to shape the beam but has the greatest limitations.
Technique 3 utilizes flat mirror segments of a fixed aspect ratio, mounted in a concave backup structure and can achieve uniform power density. However, it has the disadvantages that the ratio is not adjustable and the mirror is extremely expensive and usually requires other optics to reimage the spot to the proper absolute size.
Technique 4, which employs vibration to accomplish scanning, provides more flexibility for adjusting size and shape of the beam on the work piece. The spot size of the beam at the work piece is adjusted by changing the distance from the scanner to the work and the scan width adjustment is made at the scanner itself. Scanning can be used to create a uniform, time-averaged distribution of power, preventing melting or soft spots in the hardened surface.
Scanning using a vibrating mirror has been used with high power lasers, but the method has several problems. The principal problems have been a lack of pointing stability, variations in scan amplitude and coupling of vibrations into the scanner's mechanical support, causing further problems of pointing stability and drift and tending to weaken the integrity of the system.
It is an object of this invention to improve laser scanning, particularly for heat treating, through a system which eliminates the problems of vibrating mirror scanners while also giving greater control and flexibility for adjustment of the beam.